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F/1 Laboratory studies of the form and oscillations towards breakup of falling raindrops in the Mainz Wind Tunnel Miklós Szakáll, Subir K. Mitra, Karoline Diehl, Stephan Borrmann

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Objectives Measurement of the time averaged axis ratio of oscillating millimetre size drops with equivalent diameters (Do) above 3 mm Evaluation of the time averaged axis ratio of oscillating drops with equivalent diameters lying between 1.1 mm and 3 mm Experimental determination of the oscillation frequency of all millimetre size drops Comparison of the experimentally obtained dynamic average axis ratios against results of various drop deformation models Evaluation of the dynamic time average form of millimetre size drops after inducing oscillations through collisions

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Mainz Vertical Wind Tunnel Hauptkammer Netze Strömungsgleichrichter Kontraktionssektion Beobachtungssektion Vakuumpumpe Schalldüse Partikelfilter Heizung/Kühlung Befeuchtung/Trocknung

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The camera MotionPro X-3 Redlake Inc x512 resolution 12x12 µm pixel size 10-bit CMOS monochrom sensor 4 GB memory (~3 sec) Ethernet interface, analog VideoOut

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Sample Video I D 0 = 660 µm f = 8; 1: fps

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Sample Video II D 0 = 6.5 mm f = 8; 1: fps

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Axis ratio

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Binary evaluated picture Region of interest Raw picture Data evaluation

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Data sample f=1/Δt b/a

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Oscillation frequencies Nelson and Gokhale, 1972: J.Geophys.Res. 77, 2724 Feng and Beard, 1991: J. Atmos. Sci. 48, 1856

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Axis ratios Chuang and Beard, 1990: J.Atmos.Sci. 47, 1374

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Axis ratios – selected drops Chuang and Beard, 1990: J.Atmos.Sci. 47, 1374

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Axis ratios – selected drops Chuang and Beard, 1990: J.Atmos.Sci. 47, 1374

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Case study I a b a and b are in opposite phase

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Case study II b a b/a FFT of a, b, b/a

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Drop width (a)

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Drop height (b)

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Oscillation amplitude of drop width

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Data evaluation interpretation of the data model calculations image processing development Measurements at different turbulence levels, eg. laminar flow smaller drops (1 mm < D 0 < 3 mm) Next steps…

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